Add persistence property to set the preference for embedding of a PC object

We allow "flat" and "nested" as two options for embedding a persistable object. "nested" is only available on datastores that support such structures (JSON, MongoDB). This persistence property will set the preference (assuming the datastore supports it)

Description

We allow "flat" and "nested" as two options for embedding a persistable object. "nested" is only available on datastores that support such structures (JSON, MongoDB). This persistence property will set the preference (assuming the datastore supports it)